Dyssynchrony is the effect of the patients respiratory demands not being appropriately met by the ventilator. The patient has their own idea about how to breathe, and the machinery supporting them, instead of making breathing easier, interferes with respiration and increases the work of breathing.

Patient-ventilator dyssynchrony has occasionally appeared in the past papers. Question 11.2 from the second paper of 2017 asked for potential causes for auto-triggering during pressure support ventilation. Question 11 from the second paper of 2001 discussed the topic in a broad "what is it and what's your management" sort of way. On the other hand, Question 21 from the first paper of 2007 was weird - it discussed the reasons for apparent triggering in a brain-dead patient, which is a dyssynchrony of a sort, as it represents inappropriate auto-triggering by the ventilator.

Why is it bad?

The work of breathing increases: which is what you don't want with mechanical ventilation (remember, the point is to make breathing EASIER.)

The patient becomes distressed (the experience of being dyssynchronous with one's ventilator resembles asphyxiation)

The patient begins to cough and/or vomit, which is a sub-optimal level of comfort.

If there was an intracranial pressure problem, it will get worse with all this straining. Then, your nurse will bolus the patient with a massive amount of propofol, and their blood pressure will plummet, which does nothing to improve their cerebral perfusion.

The causes of patient-ventilator dyssynchrony

Wasted Effort: work of breathing increases because...

The mode is mandatory but the patient is awake and fighting the ventilator;

Effort is wasted when the patient tries to terminate a breath (straining to exhale against a closed expiratory valve)

Effort is wasted when the patient tries to initiate a breath (straining to inhale against a closed inspiratory valve)

The trigger is too high and the ventilator fails to supply gas when the patient demands it.

Inadequate level of support: the flow rate is too low and it does not meet patient demand.

The auto-PEEP is too high and the patient expends a lot of effort trying to defeat it

The management of patient-ventilator dyssynchrony

Increase flow cycle-off to a higher value, terminating a breath early if the patient wants shorter breaths

Decrease the flow cycle-off to a lower value if the patient requires longer breaths

Clear the airway of sputum and secretions, and ensure its patency

Increasethe sedation

Use neuromuscular blockade

Wasted effort: The mode of ventilation is mandatory; the patient wants to trigger but cannot.

Something like this was shown to the candidates in Question 11.3 from the second paper of 2017.

The patient tries to breathe, but try as they may the cold indifferent ventilator refuses to help. Instead, it blows air at them when they don't want it, and closes the valve on them when they try to take a breath.

The solution is progress the patient to a patient-triggered mode of ventilation (eg. PSV) or to sedate them more, persisting with the same mode but abolishing their respiratory drive.

Wasted effort: the trigger is not sensitive enough; the patient wants to trigger but cannot.

The patient tries to breathe, but owing to whatever patient factors they are unable to generate the effort required to deflect 2L/min of flow, or whatever your flow trigger setting is. These minor efforts may be generating some laughably small tidal volumes, but its nothing but dead space. However, it is exhausting to continue in this fashion.

The solution is to adjust the trigger to a lower setting, or sedate the patient and move to a mandatory mode. A decently low flow trigger is 0.8L/min.

Flow rate is inadequate to meet inspiratory flow demand

Something like this was shown to the candidates in Question 11.3 from the second paper of 2017.

In order to breathe comfortably, one needs a steady flow of gas, at a sufficiently high rate.
If the flow demand is not met, the patient makes an effort ON TOP of the ventilator effort.
This appears as a "scalloping" of the pressure-time curve, which reflects the fact that the patient is generating a negative pressure with their respiratory muscles while the ventilator turbine is generating a positive pressure.

The solution is to increase the flow rate. Typically, a pressure controlled mode (including PSV) delivers maximal flow at the beginning of a breath. In fact, most modern machines do this. In some machines it is possible to adjust the "ramp" of the flow curve, in which case one may be able to increase the steepness of the ramp and thereby increase the rate of flow.

Wasted effort: there is too much Auto-PEEP and it makes it harder to trigger a breath

Let us say one has a serious airflow limitation, with tightly constricted airways and hyperinflated lungs. Let us say the intrinsic PEEP in these lungs is around 10cmH2O. In order to generate a breath, one must defeat one's intrinsic PEEP. Thus, this poor chest must generate a negative pressure of 11cmH2O to get any air movement happening (to activate the flow trigger). Perhaps the machine then supports this breath with additional flow, but so what? It doesn't help in terms of reducing respiratory effort, because a breath like this has taken an enormous effort to trigger.

The solution, apparently, is to adjust the PEEP to about 80%-90% of the intrinsic PEEP. The additional work of breathing is the result of a pressure difference between the patient and the circuit. Increasing the circuit pressure decreases this pressure difference and therefore decreases the work of breathing.

Cardiac contractions cause a small amount of air movement, and in someone with a hyperdynamic ventricle and a sufficiently sensitive flow trigger these air movements can trigger ventilator breaths. The resp rate will resemble the heart rate.

Question 11.2 asked for four possible causes of auto-triggering. Including the above, the list could potentially contain the following:

Double triggering and premature breath termination

Double triggering is evidence that the ventilator has not met the patients demand for tidal volume. The typical setting is pressure support ventilation in ARDS- the lung compliance is so low that the expiratory flow trigger is reached too soon. That trigger is usually 25-30%. Changing to a lower trigger tends to prolong insufflation time, and increase the tidal volume.

The solution is to adjust the expiratory flow trigger until the desired tidal volume is achieved.

There is too much leak around the NIV mask

In order to generate the specified pressure, the ventilator continues to deliver flow. With a large leak, this inspiration can be very uncomfortable (as the ventilator delivers 70-80 litres per minute of gas into the patients face). The normal human response to such an experience is to cough, splutter and claw desperately at the mask/nurse/doctor.

One can adjust the mask, to minimise the leak.

If this does not work, one can move on to decreasing the level of pressure support (it makes sense that with less pressure there should be less leak).

If it is not practical to decrease the pressure support level, one can INCREASE the expiratory flow trigger. This will decrease the total inspiratory time, as the machine will cycle to expiration sooner, instead of blowing ridiculously to compensate for a leak. In some ventilators, one can actually adjust the inspiratory time directly.

References

Most of this information comes from only two textbooks. With "Basic Assessment and Support in Intensive Care" by Gomersall et al (was well as whatever I picked up during the BASIC course) as a foundation, I built using the humongous and canonical "Principles and Practice of Mechanical Ventilation" by Tobins et al – the 1442 page 2nd edition.